Process for the purification of a gas containing hydrogen chloride

ABSTRACT

Solid powdered reactive composition for the purification of a gas containing hydrogen chloride, the said reactive composition comprising more than 98% by weight of sodium bicarbonate and less than 2% by weight of sodium monocarbonate and exhibiting a particle size distribution defined by an average particle diameter of less than 0.050 mm and a particle size slope of less than 5. 
     The reactive composition finds an application in the purification of flue gases from the incineration of household waste.

The invention relates to the purification of gases containing hydrogenchloride.

It more particularly relates to a reactive composition based on sodiumbicarbonate which can be used for purifying a gas from hydrogenchloride.

Incineration is a technology which is increasingly necessary for thedisposal of household or municipal waste. The incineration of householdwaste is accompanied by the formation of smoke generally containinghydrogen chloride. It is imperative to remove the hydrogen chloride fromthe smoke before discharging the latter to the atmosphere.

A known process for purifying smoke containing hydrogen chlorideconsists in treating the smoke with sodium bicarbonate, so as todecompose the hydrogen chloride and to form sodium chloride.

A process has more particularly been provided in which sodiumbicarbonate is injected in the form of a powder into the smoke exitingfrom the incinerator and the smoke, thus treated, is then conveyed to afilter (Solvay & Cie, brochure TR. 895/5c-B-1-1290). In this knownprocess, the smoke is at a temperature of 260° C. at the point ofinjection of the sodium bicarbonate. The latter is used in the form of agraded powder, obtained by milling, of which 90% by weight is in theform of particles with a diameter of less than 16 μm.

In practice, the sodium bicarbonate powder used in this known processcontains sodium monocarbonate.

It has now been found that it is possible to optimize the efficiency ofthe known process which has just been described by selecting sodiumbicarbonate of well-defined quality.

The invention consequently relates to a solid powdered reactivecomposition for the purification of a gas containing hydrogen chloride,the said reactive composition comprising more than 98% by weight ofsodium bicarbonate and less than 2% by weight of sodium mono-carbonateand exhibiting a particle size distribution defined by an averageparticle diameter of less than 0.050 mm and a particle size slope ofless than 5.

In the reactive composition according to the invention, the sodiumbicarbonate and the sodium mono-carbonate are regarded as in theanhydrous state.

According to the invention, the reactive composition comprises more than98% (preferably at least 99%) by weight of sodium bicarbonate and lessthan 2% (preferably at most 1%) by weight of sodium mono-carbonate. Itcan also comprise other impurities commonly found in commercial sodiumbicarbonate, in particular sodium chloride.

The reactive composition according to the invention is a powdered solid.An important characteristic of the reactive composition according to theinvention lies in its particle size distribution which is characterizedby an average particle diameter D_(m) of less than 0.050 mm (preferablyat most equal to 0.030 mm) and a particle size slope σ of less than 5(preferably at most equal to 3).

In the reactive composition according to the invention, the averagediameter D_(m) and the particle size slope σ are defined by thefollowing relationships${D_{m} = \frac{\sum{n_{i} \cdot D_{i}}}{\sum n_{i}}},{\sigma = \frac{D_{90} - D_{10}}{D_{50}}}$

in which n_(i) denotes the frequency (by weight) of the particles withdiameter D_(i), and D₉₀ (D₅₀ and D₁₀ respectively) represents thediameter at which 90% (50% and 10% respectively) of the particles of thereactive composition (expressed by weight) have a diameter of less thanD₉₀ (D₅₀ and D₁₀ respectively). These particle size parameters aredefined by the method of analysis by diffraction of laser beams using aSympatec model Helos 12LA measuring device manufactured by SympatecGmbH.

The reactive composition according to the invention is especiallyintended for the purification of smoke originating from furnaces for theincineration of household waste. This smoke generally contains, inaddition to hydrogen chloride, sulphur dioxide. It has been observedthat the reactive composition according to the invention efficientlypurifies the smoke, both from hydrogen chloride and from sulphurdioxide.

In the reactive composition according to the invention, the sodiumbicarbonate is the main active compound. The sodium carbonate contentand the particle size distribution are, moreover, two interdependentparameters. On the one hand, it has been observed that, everything elsebeing equal, an excessive sodium carbonate content tends to cause theparticles of the reactive composition to stick together, reducing theefficiency of the composition. On the other hand, the particle sizedistribution of the reactive composition has to be a compromise, as itis known that a fine particle size will promote the reaction with thehydrogen chloride in the treated gas, whereas a coarse particle sizewill promote the subsequent separation of the solid reaction products.In practice, reactive compositions which are especially recommended arethose in which the sodium bicarbonate content by weight is from 99 to99.9%, the sodium monocarbonate content by weight is from 0.1 to 1%, theaverage particle diameter is from 0.010 to 0.030 mm and the particlesize slope is from 1 to 3. The optimum values of the particle sizeparameters are related to the means used to separate the solid productsfrom the reaction of the reactive composition with the hydrogen chlorideand, if appropriate, the sulphur dioxide in the treated gas. In fact, ithas proved to be the case that these reaction products have a particlesize distribution which is directly related to that of the reactivecomposition.

An embodiment of the reactive composition according to the invention,which is especially suited to the specific case where an electrostaticfilter is used as means for separating the abovementioned reactionproducts, is that in which the average particle diameter is from 0.020to 0.030 mm and the particle size slope is from 1.25 to 2.50. Apreferred alternative form of this embodiment of the invention is thatin which at least 90% by weight of the particles have a diameter equalto or less than 0.055 mm and at most 10% by weight have a diameter ofless than 0.006 mm.

Another embodiment of the reactive composition according to theinvention, especially suited to the specific case where a filter of thefilter-cloth type (for example a bag filter) is used as means formechanical separation of the abovementioned reaction products, is thatin which the average particle diameter is from 0.010 to 0.020 mm and theparticle size slope is from 1 to 3. A preferred alternative form of thisother embodiment of the invention is that in which at least 90% byweight of the particles have a diameter equal to or less than 0.035 mmand at most 10% by weight have a diameter of less than 0.005 mm.

The reactive composition according to the invention is generallyobtained by milling a coarse commercial sodium bicarbonate powder. Thecomposition of the powder and its particle size grading play animportant role in the quality of the reactive composition according tothe invention.

According to an especially recommended embodiment of the reactivecomposition according to the invention, the latter is obtained bymilling a powder which comprises more than 98% (preferably at least 99%)by weight of sodium bicarbonate and less than 2% (preferably at most 1%)by weight of sodium monocarbonate and which exhibits a particle sizedistribution such that at least 85% by weight of its particles have adiameter of less than 0.500 mm and such that at most 25% by weight ofits particles have a diameter of less than 0.040 mm. In accordance witha preferred alternative form of this embodiment of the invention, thepowder used for the milling exhibits a particle size distribution suchthat, per 100 units by weight of its particles, 85 units have a diameterof less than 0.250 mm, from 50 to 70 units have a diameter of less than0.125 mm, from 30 to 50 units have a diameter of less than 0.090 mm andless than 25 units have a diameter of less than 0.045 mm.

In the especially recommended embodiment which has just been described,the milling conditions are an important parameter. In particular, it isimportant to adjust the milling in order to avoid excessive or untimelythermal decomposition of the sodium bicarbonate in the powder under theeffect of the heat given off by the milling. The optimum millingparameters are directly related to other parameters, such as the millused, the particle size grading of the powder subjected to the millingand its sodium bicarbonate and monocarbonate contents, the particle sizedistribution desired for the reactive composition according to theinvention and its sodium bicarbonate and monocarbonate contents. Inpractice, the optimum milling parameters can easily be determined ineach specific case by routine laboratory work.

Everything else being equal, the reactive composition according to theinvention exhibits optimum efficiency as reactant for the purificationof smoke from hydrogen chloride and, if appropriate, from sulphurdioxide. This efficiency is revealed by the excess reactive compositionnecessary to decompose a defined fraction of the hydrogen chloride andof the sulphur dioxide in the gas, with respect to the stoichiometricamount.

The reactive composition according to the invention finds anadvantageous application in the purification of the flue gases from theincineration of household waste.

The invention consequently also relates to a process for thepurification of smoke containing hydrogen chloride, according to which areactive composition in accordance with the invention, containing sodiumbicarbonate, is introduced into the smoke at a temperature of 125 to600° C. and the smoke is then subjected to dust removal.

In the process according to the invention, the reactive composition isintroduced in the solid form into the smoke. In general, the reactivecomposition is introduced into the stream of the smoke moving within areaction chamber. In the latter, the sodium bicarbonate decomposes thehydrogen chloride of the smoke with the formation of sodium chloride.The removal of dust from the smoke has the function of extractingtherefrom the sodium chloride particles formed. It can be carried out byany known appropriate means, for example by mechanical separation in acyclone, by filtration through a filter cloth or by electrostaticseparation. In the implementation of the process according to theinvention, it is necessary to provide sufficient time for reaction ofthe sodium bicarbonate with the hydrogen chloride in the smoke, beforesubjecting the latter to dust removal. In practice, it has provedadvantageous to carry out dust removal more than 2 seconds (preferablywithin a time of 2.5 to 6 seconds) after the end of introduction of thereactive composition into the smoke.

The process according to the invention finds an especially advantageousapplication in the purification of smoke originating from theincineration of household waste. In this specific application of theprocess according to the invention, the solid product collected from thedust removal generally comprises, in addition to sodium chloride, sodiumsulphate, polyvalent metals in the metallic or combined state, andsodium carbonate. This solid product can be treated in the way set outin International Patent Application WO 93/04983 [Solvay (Soci{acute over(e)}t{acute over (e )}Anonyme)].

Distinctive features and details of the invention will emerge from thefollowing description of the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 diagrammatically shows a plant for the incineration of householdwaste which makes use of the process according to the invention;

FIGS. 2 and 3 are two diagrams which reproduce the cumulative particlesize distributions of two specific embodiments of the reactivecomposition according to the invention.

The incineration plant represented in FIG. 1 comprises an incinerationfurnace 1 (shown in part), a recovery boiler 2, a vertical tubularreaction chamber 3, a dust-removal device 4 and a chimney 5. Thedust-removal device 4 comprises a cyclone 6, followed by an array 7 ofelectrostatic filters. As an alternative, the plant can comprise afilter-cloth filter, for example a bag filter, as a replacement for ordownstream of the array of electrostatic filters 7.

The reaction chamber 3 is equipped with an injector 8 connected to adevice 14 for supplying a reactive composition.

During the operation of the plant in FIG. 1, the incineration furnace 1generates smoke laden with hydrogen chloride, with sulphur dioxide andwith various gaseous and solid impurities, including heavy metals. Thissmoke leaves the furnace 1, passes through the boiler 2, in which partof its substantial heat is recovered, and then emerges, via a flue 9,into the lower part of the reaction chamber 3, below the injector 8. Theinjector 8 is, moreover, supplied with a powdered reactive compositionin accordance with the invention originating from the supply device 14,the operation of which will be clarified below. Thus, the smoke istreated in the chamber 3 with the reactive composition originating fromthe injector 8, the result of which is the decomposition of the hydrogenchloride and the sulphur dioxide in the smoke with the formation ofparticles of sodium chloride and of sodium sulphate. On its departurefrom the reaction chamber 3, the smoke passes into the dust-removaldevice 4, where the solid matter which it contains, in particular theheavy metals and the sodium chloride and the sodium sulphate formed inthe reaction chamber 3, are separated. The purified smoke is thentransferred as is to the chimney 5.

The reactive composition used to supply the injector 8 originates fromthe supply device 14. The latter comprises a silo 10 containing acommercial sodium bicarbonate powder and a mill 11 supplied from thesilo 10 via the vibrating distributor 12. The mill 11 is of theattrition type, in which the attrition energy is imparted to thematerial to be milled via a stream of air (Kirk-Othmer, Encyclopedia ofChemical Technology, Volume 21, 1983, page 157). The mill contains asieving device. It is connected to the injector 8 via a pipe 13.

The following examples serve to illustrate the invention.

EXAMPLE 1

Household waste was incinerated in an industrial plant of the type ofthat described above, with reference to FIG. 1. On its departure fromthe incineration furnace, the smoke exhibited the followingcharacteristics:

Composition by weight (mg/Nm³): HCl: 1300 SO₂: 200 Temperature: 250° C.Flow rate: 3500 Nm³/h

Moreover, a commercial sodium bicarbonate powder was used whichexhibited the following characteristics:

to obtain a reactive composition exhibiting the particle sizedistribution represented in FIG. 2 and the following composition byweight:

NaHCO₃: >99%

Na₂CO₃: <1%

The cumulative particle size distribution of the reactive compositioncollected from the mill is reproduced in FIG. 2. In this figure, thescale of the abscissae represents the diameter of the particles,expressed in microns, and the scale of the ordinates represents thecumulative fraction by weight, expressed as % of the sieved material.

The reactive composition collected from the milling was immediatelyinjected as is into the smoke, with a flow rate substantiallycorresponding to 1.6 mol of NaHCO₃ per mole of (HCl+SO₂) in the smoke.Dust was then removed from the treated smoke on an electrostatic filter.

The composition of the smoke after the purification treatment and dustremoval was analyzed:

HCl: 20 mg/Nm³

SO₂: 10 mg/Nm³

EXAMPLE 2

The test of Example 1 was repeated with smoke exhibiting the followingcharacteristics at the outlet of the incineration furnace:

Composition by weight (mg/Nm³): HCl: 647 SO₂: 100 Temperature: 167° C.Flow rate: 36000 Nm³/h

The same commercial sodium bicarbonate powder was used as in Example 1.The milling conditions for the powder were, however, modified so as toobtain a reactive composition exhibiting the cumulative particle sizedistribution represented in FIG. 3 (the scales of the abscissae and ofthe ordinates are identical to those in FIG. 2) and the followingcomposition by weight:

NaHCO₃: >99%

Na₂CO₃: <1%

The reactive composition collected from the milling was immediatelyinjected as is into the smoke, with a flow rate substantiallycorresponding to 1.2 mol of NaHCO₃ per mole of (HCl+SO₂) of the smoke.Dust was then removed from the treated smoke on a filter-cloth filter.

The composition of the smoke after the purification treatment and dustremoval was analyzed:

HCl: 5 mg/Nm³

SO₂: <10 mg/Nm³

What is claimed is:
 1. Process for purification of smoke containinghydrogen chloride, which comprises the steps of introducing into thesmoke a powdery reactive composition comprising at least 99% by weightsodium bicarbonate and at most 1% by weight sodium monocarbonate andexhibiting a particle size distribution defined by an average particlediameter of from 0.020 to 0.030 mm and a particle size slope of from 1to 3, and subjecting the smoke to dust removal.
 2. Process according toclaim 1, characterized in that the dust removal comprises anelectrostatic separation and in that the reactive composition which isintroduced into the smoke has an average particle diameter from 0.020 to0.030 mm and a particle size slope from 1.25 to 2.50.
 3. Processaccording to claim 1, characterized in that the dust removal comprises afiltration through a filter cloth.
 4. Process according to claim 1,characterized in that the removal of dust from the smoke is carried outmore than 2 seconds after the end of introduction of the reactivecomposition into the smoke.
 5. Process according to claim 4,characterized in that the time elapsed between the end of introductionof the reactive composition into the smoke and the dust removal is from2.5 to 6 seconds.
 6. The process according to claim 1, wherein thesodium bicarbonate content by weight is from 99 to 99.9% and the sodiummonocarbonate content by weight is from 0.1 to 1%.
 7. The processaccording to claim 1, wherein the particle size slope is from 1.25 to2.50.
 8. The process according to claim 1, wherein the compositioncomprises particles of which at least 90% by weight have a diameterequal to or less than 0.055 mm and of which at most 10% by weight have adiameter of less than 0.006 mm.
 9. The process according to claim 1,wherein the composition exhibits the cumulative particle sizedistribution represented in FIG.
 2. 10. The process according to claim1, wherein the composition is composed of particles of which at least90% by weight have a diameter equal to or less than 0.035 mm and ofwhich at most 10% by weight have a diameter of less than 0.005 mm. 11.The process according to claim 1, wherein the composition exhibits theparticle size distribution represented in FIG.
 3. 12. The processaccording to claim 1, wherein the particle size distribution is obtainedby milling a powder which comprises at least 99% by weight of sodiumbicarbonate and at most 1% by weight of sodium monocarbonate and whichexhibits a particle size distribution such that at least 85% by weightof its particles have a diameter of less than 0.500 mm and such that atmost 25% by weight of its particles have a diameter of less than 0.040mm.
 13. The process according to claim 1, wherein the compositionexhibits a particle size distribution such that, per 100 units by weightof particles, 85 units have a diameter of less than 0.250 mm, from 50 to70 units have a diameter of less than 0.125 mm, from 30 to 50 units havea diameter of less than 0.090 mm and less than 25 units have a diameterof less than 0.045 mm.
 14. The process according to claim 1, wherein thesmoke is flue gas from the incineration of household waste.